Prof. Powell on the Theory of the Aberration of' Light. 429 



1729), he employed a small model (still preserved among the 

 apparatus of the experimental lecture-room), which consists 

 of a board or ruler made to move parallel to itself, while, 

 by means of.the same motion, a mark on a thread moves pa- 

 rallel to th'e ruler over pulleys attached to the fixed part, 

 and consequently describes the diagonal while the ruler moves 

 along the base*. 



That this contrivance may be spoken of as showing in ge- 

 neral the apparent direction in which the light would enter 

 the eye, is indeed obvious. But the question still remains, 

 xv/tat light is itVhich thus seems to come out of its real direc- 

 tion ? and it would seem impossible to consider Bradley's 

 model without perceiving that the mark passing down the 

 diagonal must represent the ray passing down the tube of' the 

 telescope, which is itself in motion with the observer ; and it is 

 essentially this light which is the medium of the deception, 

 since by composition of motions the diagonal actually coincides 

 with another direction in space, viz. the direct course of the 

 ray from the star: or to recur to the case of the boat, what is it 

 which corresponds to the misleading vane ? the answer could 

 only be — the telescope. 



The fuller discussions given in the most approved elemen- 

 tary treatises, in pointing more precisely to the difference be- 

 tween the real place of the star at the instant, and that shown 



* As it may not be uninteresting to preserve some notice, even of the 

 slightest ideas, of so great a man as Bradley, I subjoin a rough sketch ex- 

 hibiting the essential part of the little instrument referred to above. The 

 top and side of the box are omitted to give a view of the interior ; and it 

 will be seen at a glance that the axis (a a t ), which at once carries the screw 

 (s) (working in the teeth (qq t ) fixed to b), and on which the two ends of the 

 thread (t t,) wind in opposite directions, will by the same motion cause that 

 thread to move over the pulleys (pp.), and give the sliding-board (b) a 

 lateral motion. On its lower side (which is the part to be exhibited to the 

 spectator) is drawn a parallelogram, and its diagonal divided into equal 

 parts, so that the longitudinal motion of a mark upon the thread may be 

 clearly traced upon the diagonal as the parallelogram moves laterally. 



